Monitoring apparatus and method particularly useful in photolithographically processing substrates

ABSTRACT

An apparatus for processing substrates according to a predetermined photolithography process is presented. The apparatus includes a loading station in which the substrates are loaded, a coating station in which the substrates are coated with a photoresist material, an exposing station in which the photoresist coating is exposed to light through a mask having a predetermined pattern to produce a latent image of the mask on the photoresist coating, a developing station in which the latent image is developed, an unloading station in which the substrates are unloaded and a monitoring station for monitoring the substrates with respect to predetermined parameters of said photolithography process before reaching the unloading station. The monitoring station comprises an optical monitoring system comprising a spectrophotometric channel, and is accommodated in a sealed enclosure, such that incident light passes through the optical system towards the substrate through a transparent window.

This is a CIP of parent application Ser. No. 09/184,727, filed Nov. 2,1998, now U.S. Pat. No. 6,166,801.

FIELD OF THE INVENTION

The present invention relates to a monitoring apparatus and methodparticularly useful in photolithographically processing of substrates,particularly useful in the manufacture of semiconductor devices,

BACKGROUND OF THE INVENTION

The principal process of production of semiconductor devices isphotolithography, which includes three main serial steps or operations:

(a) coating a semiconductor wafer with photoresist material (PR);

(b) exposing the PR through a mask with a predetermined pattern in orderto produce a latent image of the mask on the PR; and

(c) developing the exposed PR in order to produce the image of the maskon the wafer.

The satisfactory performance of these steps requires a number ofmeasurements and inspection steps in order to closely monitor theprocess.

Generally speaking, prior to a photolithography process, the wafer isprepared for the deposition of one or more layers. After aphotolithography process is completed, the uppermost layer on the waferis etched. Then, a new layer is deposited in order to begin theaforementioned sequence once again. In this repetitive way, amulti-layer semiconductor wafer structure is produced.

FIG. 1 schematically illustrates a typical set-up of photocluster toolsof the photolithography process in a semiconductor fabrication plant(Fab). The photocluster (or link) is composed of two main parts: aphototrack 5, and an exposure tool 8. The phototrack includes a coatertrack 6 having a cassette load station 6 a, and a developer tack 10having a cassette unload station 10 a. Alternatively, both coater anddeveloper functions may be combined and realized in the same stations(not shown). The wafer W is placed in the cassette station 6 a. Fromthere, the wafer is loaded by a robot 2 to the coater track 6, where thecoating step (a) commences. After step (a), the wafer is transferred bythe robot 2 to the exposure toot 8, where the exposing step (b) isexecuted. Here, using optical means installed inside the exposure tool,the pattern on the mask is aligned with the structure already on thewafer (registration). Then, the wafer W is exposed to electromagneticradiation trough the mask. After exposure, robot 2 transfers the waferto the developer track 10 where the micro-dimensional relief image onthe wafer is developed (step (c)). The wafer W is then transferred byrobot 2 to the cassette station 10 a. Steps (a)-(c) also involve severaldifferent baking and other auxiliary steps which are not describedherein.

As shown in FIG. 1, the coater track 6, the exposure tool 8, and thedeveloper track 10, are tightly joined together in order to minimizeprocess variability and any potential risk of contamination duringphotolithography, which is a super-sensitive process. Some availablecommercial exposure tools are series (MA-1000, 200, 5500) of DainipponScreen MFG. Co. Ltd., Kyoto, Japan, PAS-5500 series of ASM Lithography,Tempe, Ariz., series FPA 3000 and 4000 of Canon USA Inc., USA, andMicroscan of SVGL, Wilton, Conn. Some available phototracks are series90s and 200 of SVGT, San-Jose, Calif., Polaris of FSI International,Chaska, Minn., and phototracks D-spin series (60A/80A, 60B, 200) ofDainippon Screen MFG. Co. Ltd., Kyoto, Japan, Falcon of FairchildTechnologies Inc., USA and of Tokyo Electric Laboratories (TEL.), Japan.

It is apparent that in such a complex and delicate production process,various problems, failures or defects, may arise or develop during eachstep, or from the serial combination of steps. Because of the stringentquality requirements, any problem which is not timely discovered mayresult in the rejection of a single wafer, or of the entire lot.

In modem photolithography processing, especially using DUV exposure, awafer cannot be taken out of the photocluster for measurement orinspection before the process is completed and the wafer arrives at thecassette station 10 b. As a result, any process control based onmeasuring processed wafers cannot provide ‘real time’ processmalfunction detection. Therefore, there is an urgent need for anapproach based on integrated monitoring, i.e., a monitoring apparatusphysically installed inside or attached to the relevant production unit,dedicated to it, and using its wafer handling system. Such integratedmonitoring can provide tight, fast-response and accurate monitoring ofeach of the steps, as well as complete and integrated process controlfor the overall semiconductor production process, in general, and forphotolithography, in particular.

However, the existing monitoring and control techniques typicallyutilize ‘stand-alone’ monitoring systems. A ‘stand-alone’ monitoringsystem is installed outside the production line, and wafers aretransferred from the production unit to this system using a separatewafer handling arrangement than that of the production process.

In general, three different monitoring and control processes areperformed at the present time during semiconductor fabrication process.These are monitoring of (a) overlay registration, (b) inspection and (c)critical dimension (CD) measurement. A brief description of each ofthese processes is given below:

(a) Overlay Registration Control

The overlay registration (hereinafter—“overlay”) is a process executedin the exposure tool 8 in which the pattern on the mask is aligned withrespect to the pattern features existing already on the uppermost layeron the wafer. The shrinking dimensions of the wafer's features increasethe demands on overlay accuracy.

An overlay error or misregistration (hereinafter—“overlay error”) isdefined as the relative misalignment of the features produced by twodifferent mask levels. The error is determined by a separate metrologytool from the exposure tool.

FIG. 2A illustrates a typical overlay error determination site on awafer. It is composed of two groups of target lines, one on theuppermost feature layer of the wafer 11 and the second is produced onthe new PR layer 16. Target lines 16 are similar but smaller than targetlines 11; thus they can be placed in the center of target lines 11.Therefore, these overlay targets are called “bars in bars”. FIG. 2B is atop view of the same overlay error determination site. The lines ofthese targets, such as 11 a and 16 a are typically of ˜2 μm width, and10-15 μm length, respectively.

According to a common method, the overlay error is defined as therelative displacement of the centers of target lines 11 with respect tolines 16, in both the X- and Y-axes. For example, in FIG. 2B thedisplacements between lines 11 a and 16 a, 11 b and 16 b are denoted as14 a and 14 b, respectively. Thus, the overlay error in the X-axis isthe difference between the lengths of lines 14 a and 14 b.

FIG. 3 illustrates a common configuration of photocluster tools and a‘stand-alone’ overlay metrology system composed of a measurement unitand an analysis station. It should be noted that wafers to be examinedare taken out of the photolithography process-line, and handled in themeasurement tool. This is associated with the following features of theavailable overlay technology: (i) closed loop control in ‘real time’ isimpossible; (ii) not all the wafers as well as all the layers within awafer are measured for overlay error; (iii) additional process step isneeded; and (iv) a ‘stand alone’ tool is needed. It should be noted thatit is a common situation in the Fab, especially in advanced productionprocesses, flat during ‘stand alone’ overlay measurement, the processingof the lot is stopped. This break may even take a few hours.

The results of the measurements are sent to the analysis station, and afeedback is returned to the stepper in the photocluster tool.

U.S. Pat. No. 5,438,413 discloses a process and a ‘stand-alone’apparatus for measuring overlay errors using an interferometricmicroscope with a large numerical aperture. A series of interferenceimages are at different vertical planes, and artificial images thereofare processed, the brightness of which is proportional to either thecomplex magnitude or the phase of the mutual coherence. The differencesbetween synthetic images relating to target attribute position are thenused as a means of detecting overlay error. KLA-Tencor, Calif., theassignee of this patent, sells a ‘stand-alone’ machine under the brandname KLA-5200. In this system, the measurement and the analysis stationare combined together.

U.S. Pat. No. 5,109,430 discloses another overlay metrology system. Bycomparing spatially filtered images of patterns taken from the waferwith stored images of the same patterns, the overlay error isdetermined. Schlumberger ATE, Concord, Mass., the assignee of thispatent, supplies a ‘stand-alone’ machine for submicron overlay under thebrand name IVS-120.

Other ‘stand-alone’ overlay metrology systems are manufactured byBIO-RAD micromeasurements, York, Great Britain, under the brand nameQuestar Q7, as well as by Nanometrics, Sunnyvale, Calif. (Metra series).

All the aforementioned methods and metrology systems for determiningoverlay error suffer from several drawbacks including the following:

1) They are all ‘stand-alone’ systems, i.e., operating off-line thephotolithography process. Hence, they provide post-process indication ofoverlay errors, not during the production process itself, or before abatch of wafer production is completed. In some cases this may takehours, or more.

2) They result in a waste of wafers, and/or lots of wafers because ofthis post-process response. This results from the continuous operationof the photolithographic process on the one hand, and the time-delayfrom the time a wafer is sent off-line to overlay measurements until aresponse about an error is obtained, on the other band.

3) Usually, one of the main overlay error sources is the first maskalignment for a wafer to come on a lot. Such an error source cannot becorrected later since the error varies for each lot. For this reason itis important to have the feedback within the time frame of the firstwafer which cannot be obtained using a ‘stand-alone’ tool.

4) The overlay sampling frequency is limited due to contaminationrestrictions and additional expensive time needed for extra handling andmeasurements.

5) Throughput of the photolithography process is reduced as a result ofthe post-process overlay detection and the long response-time, as wellas of the reduced sampling frequency mentioned in (3).

6) These stand-alone systems require additional expensive foot-print andlabor in the Fab.

7) The microlithography tools are the “bottle neck” in thesemiconductors production process and they are the most expensive toolsin the FAB. Their partial utilization due to late off-line measurementsreduces drastically overall equipment efficiency in the Fab.

(b) Inspection

Inspecting during the production of semiconductors wafers can be definedas a search for defects caused by:

(a) contamination (dirt, particles, chemicals, residues, etc.), and/or

(b) process induced failures related to PR, coating, baking,development, handling, etc.

In order to detect defects originating only from the lithographyprocess, a specific inspecting step is conducted after the developmentstep as illustrated in FIG. 4. It is usually called “after developmentinspection” (ADI), or “post-development check” (PDCK). The presentinvention is mainly relevant for ADI.

In general, data obtained during the inspecting is analyzed, and in casean increased defects level is detected, an alarm is sent to theengineering level or to the production line. Once again it should benoted that, as in the case of overlay metrology, with the currenttechnology, the ADI is located out of the production line; i.e., wafersto be inspected are taken out of the production process and handled in aseparate inspecting station. It should also be noted that it is a commonsituation in the Fab, especially in advanced production processes, thatduring ‘stand alone’ inspecting, the processing of the lot is stopped.This break may take even few hours.

Today, the majority of ADI activities are non-automatic visualinspecting conducted by humans. In particular, no integrated automaticADI system is commercially available at the moment.

ADI is aimed at:

(i) Coarse inspecting—A wafer is handled by hands and is visuallyinspected by eye-sight for large defects. These defects can be, forexample, poor spinning during coating, poor development, scum,non-attached PR (‘lifting’), and/or edge beads. This method can usuallyonly detect defects bigger than tens of microns.

(ii) Fine inspecting—predetermined sites or targets on a wafer arevisually inspected with the aid of a microscope (20-50×magnification).

These defects can be, e.g., shorts between conducting lines, and focusfailures of the stepper.

ADI conducted by humans has several disadvantages:

(a) It is tedious and requires great concentration to locate patterndiscrepancies in repetitive and complex circuits.

(b) The results are not uniform with respect to each inspector as wellas between different inspectors. This point becomes crucial whenconsidering the increased importance of inspecting at times when thewafer features become more and more delicate due to the continuousshrinking of the wafer's features.

(c) It is not a consistent means for statistical analysis and formeasuring process quality due to non-repetitive results.

(d) Additional costs due to the labor.

(e) Non objective inspecting, neither in defects identification nor inthe specific action which should take place once a specific defect isidentified.

(f) Fluctuating throughput results, among other things, in difficultiesto determine sampling frequency.

(g) Manual inspecting is also done off-line and therefore suffers fromall the same aforementioned disadvantages of ‘stand-alone’ systems.

To complete the picture, it should be noted that two automatic opticalinspection (AOI) methods for defect detection are known, but their highcost and low throughput limit their use in actual production.

(i) Absolute methods—illuminating a wafer at a predetermined angle(“grazing”) and collecting the reflected signal from the wafer's plane.Any signal above a threshold (absolute) value is determined as a defect.According to this method, particles bigger than 0.1 μm can be detected.

(ii) Comparative methods—these are divided into ‘die to die’ and ‘die todatabase’. A wafer is photographed and then an automatic comparison ofpixels in one die is made with respect to the correlative pixels in aneighbor die, or to a database. Usually, the result of the comparisonshould fit a set threshold, unless there is a defect. The threshold maybe a function of the gray level and/or the specific location of the dyein the wafer.

Method (ii) overcomes the shortcomings of method (i), and usuallydetects defects such as dirt particles (>0.1 μm), bridging of conductinglines, missing features, residues of chemicals and PR, etc. The defectlevel these methods can detect is determined according to the designrule of the industry (e.g., 0.1 μm).

None of the available inspecting tools samples each wafer, but onlyseveral wafer in a lot. Moreover, the lack of such inspecting systemsprevents any option for automatic and tight feedback or closed loopcontrol over the lithography process. Thus, any serious attempt forestablishing or even improving the process control around thephotolithography process is prevented, or at least is met with crucialobstacles due to the lack of such method(s) and systems.

(c) Critical Dimension (CD) Control

A third monitoring and control process is the Critical Dimension CDcontrol which includes measurements of characteristic dimensions incritical locations on a wafer, e.g., widths of representative lines,spaces, and line/space pairs on the wafer. CD metrology tools are basedon two main technologies: the CD scanning electron microscope (CD SEM),and the atomic forced microscope (CDAFM). Commercial tools based on CDSEM are series 7830XX of Applied Materials, Santa Clara, Calif., andDEKTAK SXM-320 of VFEFCO, USA is based on AFM.

FIG. 5 illustrates common configurations of ‘standalone’ CD tools withthe production process. Typically, CD measurements take place after thedeveloping step and/or after etching. The CD tool is located out of theproduction line, i.e., wafers to be measured are taken out of theproduction process and handled to a separate CD station. It should benoted that it is a common situation in the Fab, especially in advancedproduction processes, that during ‘stand alone’ CD measurement, theprocessing of the lot is stopped. This break may take even few hours.

In general, data obtained during the CD measurement is analyzed, andthen a kind of feedback (or alarm in a case of a width out of thepermitted range) is sent to the relevant unit in the production line.

CDSEM and CDAFM allow CD measurement for line/space width below theresolution limit of optical microscope. However, when possible, opticalCD (OCD) measurement may be very useful because they can be combinedwith optical overlay measurement systems. Recently, (C. P. Ausschnitt,M. E., Lagus (1998) Seeing the Forest for the Trees: a New Approach forCD Control”, SPIE, vol. 3332, 212-220), it was proposed to use OCD evenfor sub-micron design rules that is behind the optical resolution. Theidea is that optical systems allow fast measurement of many liessimultaneously. Statistical treatment of multiple measurements with lowaccuracy, allows to extract such important manufacturing data asrepeatability or deviation trends.

It is clear, as was noted before with respect to overlay metrology andinspecting tools, that since all CD metrology systems are ‘stand-alone’tools, they suffer from the same drawbacks as discussed before.Moreover, especially in the case of CD measurement, the results, e.g.,line width, give a limited ability to correlate the measurement to anyspecific cause.

Methods for Process Control in Lithography

Overlay and CD monitoring can be performed in various levels in order toestablish process control. The first common level is “lot to lotcontrol”. In this method each lot is a basis for the next lot to run inthis process. Small correction can be made by considering the results ofthe previous lot and making corrections. However a certain increment inthe risk is introduced because a total lot may be lost.

A second control level is “send ahead wafer”. In this method a pilotwafer is sent through the coating-exposure-developing steps, exposed inthe recommended exposure, and is then sent to CD measurement.Satisfactory results will be a basis for the set up conditions of thelot, whereas unsatisfactory results will cause another wafer to beexposed with corrections for the exposure conditions. The over allsequence of a “send ahead wafer” control can take many hours whilevaluable utility time of the production tools, as well as the productionlot may be lost.

In some cases there is a need for a higher control level. This may beperformed in a full process window mapping by running an exposurematrix, or focus exposure matrix, and analyzing the results. However,this is the most time-consuming method.

The drawbacks of these methods, when conducted with ‘stand-alone’overlay and OCD tools, are that they are time and effort consuming andthey usually do not respond directly for certain causes, or do notreveal any problematic sources. However, they make the “time to respond”shorter as compared to long-term trend charts. Nonetheless, to enable areal feedback to problems, there is a crucial need for integratedmonitoring of the process steps. The on-line measurements can responddirectly to a certain cause with the correct straight forward correctionaction.

It should be emphasized that these problems with respect to processcontrol ‘stand-alone’ systems are dramatically aggravated whenconsidering the coming future developments in the semiconductorindustry. Because of the shrinking critical dimensions of the wafer'sfeatures, as well as the introduction of new and non-stable processes(e.g., DUV resist, and transition to 300 mm diameter wafer withcorresponding restrictions on wafer handling), the need for anintegrated monitoring and process control for semiconductors productionbecomes crucial. For this reason, traditional process control methodsthat use long-term trend charts, and which are “offline methods” will bemore and more excluded.

As noted before, integrated monitoring and process control systems are areasonable solution for the above-discussed problem. However, such asystem should be considered from several aspects and meet specificrequirements in order to become real and feasible:

(a) Small footprint—such a system should have as small footprint aspossible (practically not smaller tan the wafer size) in order to bephysically installed inside the photocluster;

(b) Stationary wafer—the wafer should be stationary during inspectionand measurement to exclude extra wafer-handling and particlesgeneration;

(c) High throughput—the system should have high throughput such as notto reduce the photocluster throughput;

(d) Cleanliness—the measuring unit should not interfere in any way withthe photocluster or introduce any potential risk of contamination;

(e) Access for maintenance—the system parts except the measuring unit(e.g., control electronics, light source), should be outside thephotocluster in order to enable, among other things, easy and quickmaintenance without any disturbance to the photocluster;

(f) Cost-effective the integrated tool cost should be a small portion ofthe phototrack cost.

“Stand-alone” monitoring and process control systems do not meet thesestringent requirements, and apparently cannot be used as an integratedsystem. Moreover, no such integrated system is now available on themarket. Therefore, there is a need for a new monitoring and processcontrol apparatus and method having advantages in the above respects.

OBJECT AND BRIEF SUMMARY OF THE PRESENT INVENTION

An object of the present invention is to provide a novel apparatus andmethod having advantages in one or more of the above-described respects,particularly important in the photolithography processing of substrates,e.g., semiconductor wafers.

According to one aspect of the present invention, there is providedapparatus for processing substrates according to a predeterminedphotolithography process, comprising:

a loading station in which the substrates are loaded; a coating stationin which the substrates are coated with a photoresist material;

an exposing station in which the photoresist coating is exposed to lightthrough a mask having a predetermined pattern to produce a latent imageof the mask on the photoresist coating;

a developing station in which the latent image is developed;

an unloading station in which the substrates are unloaded; and

a monitoring station for monitoring the substrates with respect topredetermined parameters of said photolithography process before beingunloaded at the unloading station, said monitoring station comprising:

a supporting assembly for receiving substrates to be inspected;

a sealed enclosure having a transparent window aligned with and facingsaid supporting plate, said supporting plate being externally of saidsealed enclosure and spaced from said window thereof; and

an optical monitoring system within said sealed enclosure for inspectingsubstrates on said supporting plate via said transparent window, whereinthe optical monitoring system is associated with at least one lightsource and comprises a spectrophotometric measurement channel.

As will be described more particularly below, the optical monitoringsystem between the developing station and the unloading station maydetect one or more of the following: (a) overlay registration errors;(b) inspection of substrates for defects in the photoresist layer (i.e.,estimation of the quality of the PR pattern); (c) critical dimensionalerrors; (d) measurement of the substrate's parameters (such as thicknessof at least one layer of the substrate and/or optical parameters of atleast one layer of the substrate).

In the described preferred embodiments, the at least one light source isexternally of the sealed enclosure and produces a light beam which isapplied to the optical system within the sealed enclosure.

In addiction, the optical monitoring system within the sealed enclosureincludes an optical imaging device; and the monitoring station furtherincludes a digital image processing unit accommodated externally of thesealed enclosure and connected to the optical imaging device byelectrical conductors passing into the sealed enclosure.

The monitoring station further includes a central processing unitaccommodated externally of the sealed enclosure and connected to theoptical monitoring system for controlling the system via electricalconductors passing into the sealed enclosure.

According to still further features in the described preferredembodiments, the optical monitoring system within the sealed enclosureincludes the following components: a low-magnification channel foraligning the optical inspecting system with respect to a patternedsubstrate on the supporting plate or for coarse inspection; and ahigh-magnification or high-resolution channel for measuring thepredetermined parameters of the photolithography process after thesubstrate has passed through the developing station and before reachingthe unloading station. The low-magnification channel and thehigh-resolution channel are fixed with respect to each other. Thesechannels may be associated with separate light sources.

The spectrophotometric channel may utilize a separate optical systemthat may and may not utilize a separate light source. Such an opticalsystem includes an objective lens, a beam splitter for separatingincident light and light coming from the substrate, an imaging lens anda spectrophotometer. Alternatively, an optical system of thespectrophotometic channel may utilize an objective lens, a beam splitterand an imaging lens of the low-magnification channel.

According to another aspect of the present invention, there is provideda monitoring apparatus for optically monitoring articles, comprising:

a supporting plate for supporting the article to be monitored;

a sealed enclosure having a transparent window aligned with and facingsaid supporting plate, said supporting plate being externally of saidsealed enclosure and spaced from said window thereof;

an optical monitoring system within said sealed enclosure for inspectingthe article on said supporting plate via said transparent window, theoptical monitoring system comprising a spectrophotometric channel; and

a light source for illuminating the article via said optical monitoringsystem, said light source being externally of said sealed enclosure andproducing a light beam which is applied to said optical monitoringsystem within said sealed enclosure.

The invention also provides a novel method of processing substratesaccording to a predetermined photolithography process. The methodcomprises the following operations: coating the substrate with aphotoresist material; exposing the photoresist coating to light througha mask having a predetermined pattern to produce a latent image of themask on the photoresist coating; and developing the latent image; and ischaracterized in monitoring the substrate with respect to predeterminedparameters of the photolithography process after the substrate has beendeveloped, said monitoring including spectrophotometric measurements,and controlling said photolithography process in accordance with theresults of said monitoring operation.

As will be described more particularly below, the invention permits oneor more or the following to be provided:

1) An integrated apparatus for overlay metrology and/or inspection,and/or OCD measurements. Such an apparatus would have high accuracy andhigh throughput and could be physically combined inside the present footprint of photocluster tools; i.e., it would have a zero additional footprint on the production floor. The combination of up to three differentfunctions in one tool would have its own advantages: (i) Betterexploitation of utilization time —each inspecting, overlay and CDmeasurement could have its own sampling frequency and need not be thesame for every wafer. Thus, such an apparatus could be continuouslyoperated while its utilization time is shared between the threefunctions. (ii) A direct result from (i) is that the apparatus coulddrastically decrease the numbers of lots which would be simultaneouslyrunning around the production process as common today (one for overlaymeasurement, one for inspecting, one for CD, and one which begins thelithography process), (iii) Such apparatus could be client-oriented,i.e., could be exactly fitted to the customer needs as well as tochanging needs. (iv) The apparatus could be oriented for a specificproblem; (v) The apparatus could have a relatively low price.

2) An apparatus as in (1) which will not reduce the throughput of thephotocluster by carrying out measurements/inspection in parallel withthe processing of the next wafer.

3) A modular apparatus as in (1) capable to perform only overlaymetrology or inspecting or OCD measurements with enhanced performance ifneeded.

4) An apparatus as in (1) combined with a processing unit, thus enablingto establish monitoring and process control based on overlay, inspectingand OCD measurements.

5) An integrated automatic inspecting system which is much moreaccurate, faster, and repetitive than visual inspecting conducted byhumans.

6) A method for integrated monitoring and process control for overlaymetrology and/or inspection and/or OCD within the photolithographyproduction process, thereby enabling much shorter response times ascompared to the common ‘stand-alone’ systems.

7) Methods which facilitate and dramatically shorten the time needed forcommon process control methods such as ‘send a wafer ahead’, ‘lot tolot’, etc.

8) A new and practical monitoring and process control method by means of‘wafer to wafer’. Such a method is practically impossible to conduct inthe current situation of ‘stand-alone’ systems. However, with these newintegrated methods, which do not decrease the throughput of theproduction process a tight, fast-responding, directly to cause,monitoring and process control method can be conducted. Apparently, incertain circumstances, this new method may save the need for separateand expensive ‘stand-alone’ systems for overlay metrology, inspectionand OCD.

9) A method with which both overlay error, inspection and OCD can bedetermined either on one wafer of a lot or on different wafers in thesame lot.

10) An integrated and elaborated inspecting monitoring and controlsystem.

11) The process can be implemented in several alternative levels. Onewould be aimed at notifying the production process controller aboutdefects in general. Another would be aimed at investigating the directreasons and/or the induced failure which caused the defects. Then, afeedback would be directed, to any predetermined relevant point in theproduction process. By this, a feedback or a closed loop control can beestablished either for a single step in the production process (e.g.,exposure step, post exposure bake (PEB)) or for the whole process whencombined with other metrology system(s), such as overlay and/or criticaldimension metrology systems.

Such a method and apparatus have the potential to save expensiveutilization time (e.g., by shortening methods such as ‘send a waferahead’) as well as diminishing the amount of test wafers wasted duringthe production process.

In addition, there would be no need for additional wafer handling fromor to the photolithography tools, thus saving utilization time as wellas preventing additional contamination and wafer breakage.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is typical apparatus set-up of photocluster tools as presentlyused in a photolithography process of a semiconductor fabrication plant;

FIGS. 2A and 2B are schematic illustrations of an overlay target;

FIG. 3 is a block diagram illustrating a common configuration ofphotocluster tools and a ‘stand-alone’ overlay metrology system;

FIG. 4 is a block diagram illustrating possible points where afterdeveloping, inspecting can be introduced with respect to semiconductormulti-layer production process;

FIG. 5 is a block diagram illustrating common configurations of‘stand-alone’ CD tools presently used in the production process;

FIG. 6 is a schematic illustration of one manner m which thephotolithography cluster tools in the apparatus of FIG. 1 may becombined with an integrated overlay/inspecting/OCD tool in accordancewith the present invention;

FIG. 7 is a schematic side view of an integrated lithography monitoringsystem in the apparatus of FIG. 6;

FIG. 8 is a schematic illustration of a preferred embodiment of thepresent invention used as an integrated overlay metrology tool;

FIG. 9 is a schematic illustration of one form of optical system thatmay be used in this apparatus of the present invention;

FIG. 10 is a schematic illustration of one manner that may be used formeasuring the angle between the incident ray of the high-resolutionchannel to the wafer surface;

FIG. 11 is a view along the section lines A—A in FIG. 2B;

FIG. 12 is a schematic illustration of the gray level of target lines 11a and 16 a (FIG. 11);

FIGS. 13A and 13B are schematic illustrations of the gray levels ofimages of a ‘knife edge’ pattern and their line spread function (LSF)functions;

FIG. 14 is a schematic flow chart of a method to determine overlay errorin accordance with a preferred embodiment of the present invention;

FIG. 15 is a schematic illustration of a preferred embodiment of thepresent invention as an inspecting tool;

FIG. 16 is a schematic illustration of one way by which light from alight source may be conveyed to the ring light;

FIG. 17 is a schematic flow chart of a method for both coarse and fineinspecting in accordance with a preferred embodiment of the invention;

FIG. 18 is a schematic illustration of the field of view of the opticalhead during OCD measurement;

FIG. 19 is a schematic illustration corresponding to that of FIG. 9, butshowing another form of optical system that may be used in the apparatusof the present invention;

FIG. 20 is a schematic illustration, corresponding to FIG. 10, butshowing a modification that may be included when the optical system ofFIG. 19 is used; and

FIGS. 21A and 21B are schematic illustrations corresponding to that ofFIG. 19, but showing two different examples, respectively, of an opticalsystem that may be used in the apparatus of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is especially useful in the manufacture of semiconductordevices, and is therefore described below with respect to thisapplication.

FIG. 6 schematically illustrates an apparatus set-up corresponding tothe conventional one illustrated in FIG. 1, but modified to incorporatean integrated lithography monitoring (ILM) system in accordance with thepresent invention.

Thus, FIG. 6 illustrates an apparatus for processing substrates, in thiscase wafers W, according to a predetermined photolithography process.This illustrated apparatus comprises: a loading station LS correspondingto the cassette loading station 6 a in FIG. 1, in which the cassettesare loaded onto the phototrack 5; a coating station CS, corresponding tocoater track 6 in FIG. 1 in which the wafers are coated with aphotoresist material; an exposure station ES, occupied by the exposuretool 8 in FIG. 1, in which the photoresist coating is exposed to lightthrough a mask having a predetermined pattern to produce a latent imageof the mask on the photoresist coating; a developing station DS,corresponding to developer track 10 in FIG. 1, in which the latent imageis developed; and an unloading station US, corresponding to station 10 ain FIG. 1, in which the cassettes are unloaded.

In accordance with the present invention, the apparatus illustrated inFIG. 6 is modified to include a monitoring station MS preferably betweenthe developing station DS and the unloading station US. The monitoringstation MS is occupied by an optical monitoring system, generallydesignated 14 in FIG. 6, which is an ILM (integrated lithographymonitoring) system, for measuring and/or inspecting the W with respectto predetermined parameters of the photolithography process after thewafers have passed through the developing station DS and before reachingthe unloading station US.

As will be described more particularly below, the ILM system 14 inspectsthe wafers W, immediately after processing, for one or more of thefollowing conditions after the wafers have passed through the developingstation DS and before reaching the unloading station US; (1) overlayregistration errors, in the alignment of the developed image produced onthe wafer in the respective photolithography process with respect to adeveloped image produced on the wafer in a preceding photolithographyprocess performed on the wafer, as described above in FIGS. 2a and 2 b;(2) defects in the substrate, induced by the process malfunctions, aswell as dirt particles, etc., as described above; and/or (3) criticaldimension errors in the developed image of the photoresist coatingprocess produced during the respective photolithography process, as alsodescribed above. Preferably, the ILM system 14 performs all three of theabove functions, but in some applications, it may detect only one or twoor the above type errors.

The exact location of the LM system 14 in the photocluster is governedby local considerations and circumstances, e.g., on the specificphotocluster tool manufacturer, the available foot-print inside thephototrack, and the Fab considerations.

FIG. 7 is a side view of the ILM system 14 according to a preferredembodiment of the present invention. The ILM system includes a rigid andstable supporting means 20 which receives and holds the wafer Wstationary. This can also be a vacuum supporting plate or a vacuumhandler (not shown) which holds the wafer W stationary from its bottom(back) side. Supporting plate 20 is preferably located between thedeveloping station DS and the unloading station US.

The ILM system 14 further includes a measuring unit (MU) 22 locatedabove supporting means 20. The measuring unit 22 and supporting means 20are rigidly mounted together in any suitable manner. As shown in FIG. 7,measuring unit 22 includes a sealed enclosure 21 with a transparentoptical window 37 aligned with and facing the supporting plate 20 andoptical unit inside the sealed enclosure 21, schematically indicated at23 having a movable optical head 24. The wafer is illuminated via asource 32 which is externally of the sealed enclosure 21 and directs abeam to the measuring unit 22 via an optical fiber (not shown) passinginto the sealed enclosure 21. As will be described more particularlybelow, movable optical head 24 enables the measuring unit 22 to performany one of a number of preselected measurements on any preselected waferW supported by plate 20 via transparent window 37.

As further shown in FIG. 7, the ILM system 14 further includes a controlunit generally designated 26, which is externally of the sealedenclosure 21 and is connected to the MU 22 by electrical conductors (notshown) passing into the sealed enclosure 21. Control unit 26 includes acentral processing unit (CPU) 28, and optionally, an image processingunit (IPU) 30, as well as the electronic controls (not shown) forcontrolling the real time operation of the measuring unit 22.

Thus, in accordance with this preferred embodiment, the design of theILM system 14 should meet several principles, including: (a) small size,(b) maintaining the wafer stationary during measurement, (c) rigid andstable measuring unit, (d) cleanness restrictions attained by, amongother things, full separation of measuring unit 22 from the photoclusterenvironment i.e., all moving parts are located within the sealedenclosure 21 of the unit 22 and external light source, (e) high speedmeasuring (e.g., fast scanning), and (f) easy and quick maintenance by,e.g., simple replacement of any one of the above mentioned units. It isalso noted that the ILM system has the option to be bypassed by theproduction process and to be simultaneously operated in off-line or inintegrated modes.

FIG. 8 is a schematic illustration of the ILM system 14 according to apreferred embodiment of the present invention for measuring overlayregistration errors. However, it should be noted that the hereinafterdescription is applicable as well to other preferred embodiments of thepresent invention such as an apparatus for defect inspecting, or formeasuring OCD errors. The measuring unit 22 is shown in a measuringposition, i.e., above a wafer W with a developed PR coating 36. Theoptical head 24 is able to move rapidly along X and Y axes of an X-Ystage 38, and also along the vertical Z-axis. Between the optical head24 and the wafer W there is the optical window 37 which prevents anypotential disturbance or contamination to the photocluster tools fromthe measuring unit 22.

The measuring unit 22 further includes a calibrating unit 40 whichstimulates a measuring position for the optical head 24 when it islocated above it. The calibrating unit 40 is composed of a target 42, aglass plate 44, and a mirror 46. The target 42 is any high contrastobject, such as a metallic pattern on a glass substrate which issuitable for determining the line spread function of the optical system(e.g., a knife-edge pattern). The glass plate 44 is of the same materialand thickness as optical window 37. The target is located in the objectplane of objective 76 similar to where the wafer W is located.

FIG. 9 is a schematic illustration of the measuring unit 22 according toa preferred embodiment of the present invention as an overlay metrologytool. However, the illustrated optical configuration is applicable aswell to other preferred embodiments of the present invention, such asfor defect inspecting, or for OCD metrology, in a manner to be discussedlater. As shown, the measuring unit 22 is composed of two alternativechannels inside the sealed enclosure 21: (a) an alignment orlow-magnification channel 62, and (b) a measuring or high-magnificationchannel 64. The low-magnification channel 62 is aimed at locating theoptical head at the right position above an overlay target (FIG. 2) tobe measured, whereas the high-resolution channel 64 is aimed at imagingthe overlay target. In this embodiment, a single external white sourcelight 32 and a single area CCD camera 92 serve both channels.

In another preferred embodiment of the present invention and for certainapplications, a filter(s) is added (not shown) after light source 32 inorder to produce a certain narrow spectral bandwidth which increases thecontrast of the features to be measured.

The low-magnification channel 62 comprises an objective 66, a beamsplitter 68, a shutter 70, a tube lens 72 and a beam splitter 74.Channel 62 has a relatively low-magnification power (e.g. 0.3-1.0×). Theobjective 66, which is part of the optical head 24 (FIG. 8), has a smallnumerical aperture and images a wide field of view (FOV) (e.g., 20-40mm).

The high-magnification channel 64 comprises a vertically movableobjective 76 which is part of the movable optical head 24 (FIG. 8), abeam splitter 77, a shutter 80, a tube lens 81, a beam splitter 90, afocus target 79, and LED illuminator 91. This channel has a relativelyhigh-magnification power (e.g., 20-100×). The objective 76 has largenumerical aperture since high resolution is needed and images arelatively small FOV (about 100 μm).

If higher accuracy is needed, measurement data correction may beachieved by determination of the actual incident angle of theilluminating light on the wafer's surface as illustrated in FIG. 10. Themeans for doing this is installed in the movable optical head 24 insidethe high magnification channel 64 and comprise an LED 93, two identicalmirrors 94 a and 94 b, two identical lenses 96 a and 96 b, and aposition sensor (electronic) device 98 composed of single suitablephotodiode or array of such photodiodes. The light from the LED 93 isreflected from the mirror 94 a and is focused by lens 96 a on the waferat the same location where he light from the objective 76 is focused.From there, it is brought back through lens 96 b and mirror 94 b to theposition sensor device 98. The position where the ray impulses theposition sensor device 98 is translated by means of a function to theangle β between the objective's chief ray 99 and the ray 97. Themeasured angle β is introduced during a later step of image processingin order to correct the inaccuracies which may arise during measurement.

The focusing target 79 (FIG. 9.) is any high contrast object, such as ametallic pattern on a glass substrate. The pattern can be any easilyidentifiable pattern, such as a contrast edge, a grid, etc. It isinstalled in the optical path with the option of removing it when notneeded (movable target), or of locating its image in the CCD plane 92 insuch a way which does not interfere with the imaged wafer or imagedoverlay targets. Other methods of focusing sensors can be applied aswell.

A selection is needed to enable selection between operating thealignment channel 62 or the measuring channel 64. In this embodiment theselection is realized by shutters 70 and 80 which can be selectivelyopened or closed.

Reference now is made to FIG. 11, which is a side view along the sectionline A—A in FIG. 2B. FIG. 11 illustrates the uppermost features layer100 on the wafer, and above it the developed top PR layer 102. Layers100 and 102 are separated by the interface 101, whereas layer 100 isseparated from the layer below it (not shown) by interface 103. Layer100 comprises the overlay target lines 11 a and 11 b, whereas PR layer102 comprises the overlay target lines 16 a and 16 b.

The focusing procedure is aimed at locating, in a repetitive way, theobject plane 104 of objective 76 of the measuring channel 64 atpredetermined distances, Δz₁ and Δz₂, from interfaces 101 and 103,respectively. These distances are determined during the measurementprogram reparation for a certain product to be measured.

The focus condition of objective 76 over interface 101, indicated as z₁,in FIG. 11, is determined according to any known procedure, such as thatdisclosed in U.S. Pat. No. 5,604,344. In the same manner the objective76 is additionally moved down in order to detect, in the same procedure,its exact location z₂ above interface 103. It should be noted that whenthe overlay target 17 on layer 100 is a ‘negative’ feature (e.g., atrench), instead of ‘positive’ feature such as 11 a, it is possible todetermine the focus plane 104 with respect to plane 17 a instead ofinterface 103.

Once the locations z₁ and z₂ are known, the object plane 104 ofobjective 76 can be precisely located at distances Δz₁ and Δz₂ frominterfaces 101 and 103 respectively. At this location, measuring takesplace in order to produce an approximately equivalently defocused imageof both target lines 11 and 16 onto the CCD's image plane 92.

To calculate the overlay error, the exact locations of the centers oftarget lines 11 a, 11 b, 16 a, and 16 b should be determined. For thispurpose, several alternative methods are known. It is noted that withrespect to other types of overlay targets (e.g., multi-layer box, notshown) the same below-described methods can be used. One method isillustrated with the aid of FIG. 12 which shows the gray level of targetlines 11 a and 16 a (FIG. 11) images. The gray levels are obtained bytransforming the electrical signals of the CCD camera 92 (FIG. 9) intothe digital form, e.g., by means of analogue-to-digital converter (notshown). The Central Processing Unit (CPU) 28 (FIG. 7) determines thecenters of the gray levels lines 11 a and 16 a. The difference betweenthese centers Δx expresses the length of line 14 a (FIG. 2), dependingon the magnification along the measuring and imaging channels. In asimilar manner the length of line 14 b is determined and the overlayerror can be calculated for the x axis.

In the same manner, the overlay error can be calculated for the y-axis.

When the shapes of the gray levels 11 a and 16 a (FIG. 12) are notsymmetrical with respect to the vertical axis, or are imperfect, theoverlay error may be calculated using the line spread function (LSF) ofthe measuring channel. The LSF is accurately determined with the aid ofthe calibrating unit 40 (FIG. 8) at different heights above thecalibrating target 42, FIG. 13 illustrates at “A” the gray levels ofimages of a ‘knife edge’ pattern on a calibrating target at twodifferent heights 21 and 22 above the calibrating target 42. Thederivatives of the gray levels 21 and 22 with respect to the x-axis areshown at “B”, as 23 and 24, respectively, in FIG. 13. Now, by applying ade-convolution process the CPU 28 calculates the shape of the targetline along the x axis. In order to compensate for the physical height ofthe target lines, de-convolution is conducted in at various locationsalong the target line profile (vertical axis) using the suitable LSF, asshown at “B” in FIG. 13 for each location. The target shape isdetermined from these profiles.

In the same manner, the shape of the target lines along the Y axis canbe determined, and the overlay error may be calculated.

In general, when the gray levels shapes 11 a and 16 a (FIG. 12) are notsymmetrical with respect to the vertical axis, or are imperfect, a morecomplicated algorithm can be used, e.g., a comparison of the obtainedgray levels of the target lines with their original shape and dimensionsin the masks.

FIG. 14 is a schematic flow chart of a method to determine overlay errorin accordance with a preferred embodiment of the present invention.After a new wafer to be measured arrives at the supporting plate 20(FIG. 7), calibrating of the measuring system takes place and then thewafer is aligned with respect to its principle axis. After alignment,the optical head 24 moves to a pre-determined site on the waferaccording to a previously prepared program. The program contains datawhich is relevant for operating the alignment 62 and measuring 64channels (FIG. 9), such as recognized patterns of the overlay targetonto the wafer and its coordinates. With the aid of the wide FOV of thealignment channel 62 (FIG. 9), and relevant data in the program, theoptical head 24 is brought above the site area. Now, a final alignmentcommences in order to locate the optical head 24 in its exact positionabove the site. An example of a method for wafer alignment (practicallyachieving both objectives of pre and fine alignments) based on itspattern features is disclosed in U.S. Pat. No. 5,682,242. Then, theshutter 70 (FIG. 9) enters the optical path and blocks the alignmentchannel. An autofocusing mechanism in the optical head 24 focuses theobjective 76 (FIG. 9) on the focus plane 104 (FIG. 11). The overlaytargets are imaged on the CCD 92, and the data which was obtained isprocessed by the image processing unit 30 (FIG. 7) in order to determinethe overlay error. If all the predetermined sites on the wafer arealready measured, the wafer is released back to the phototrack 5, and anew wafer is brought to the supporting plate 20. If not, measurement ofthe next site on the wafer takes place.

It is noted that the overlay too has various operational modes: (i)overlay error measurement; (ii) the same as in (i), and anothermeasurement when the wafer is rotated 180°; (iii) the same as in (i)conducted on one wafer, and another measurement conducted on anotherwafer which is rotated 180° with respect to the first; (iv) overlayerror is measured at different heights, and accuracy is determined byrotating the wafer.

According to another preferred embodiment of the present invention, theoverlay error data which is determined by the processing unit, istransferred to a general control unit 200 (FIG. 14) of the photocluster,or of a specific tool in the photocluster. General control unit 200 usesthis data for a feedback closed loop control to the exposure tool 8. Itcan also instruct the overlay metrology itself with respect to itsoperation (e.g., sampling frequency, site number to be measured on awafer).

It will be appreciated that, by combining the processed data from theoverlay system, with data of the defect inspecting process and of theOCD metrology, all within the same apparatus, an extensive integratedmonitoring and control system for the photolithography process can beestablished. It will also be appreciated that overlay errors, defectsand OCD errors can be determined during the production process itself,or after or before any predetermined step; and that all this can be doneeither on all wafers of a lot, or on several selected wafers in the samelot.

FIG. 15 illustrates a preferred embodiment of the present inventionconfigured as a defect inspecting tool.

The defect inspecting configuration is composed of: (a) two alternativeoptical channels, namely (1) a coarse inspecting channel 62 with a0.3-3.0× magnification, and a (2) fine inspecting channel 64 with >20×magnification; (b) a fast image acquisition system 320; and (c) aprocessing unit 26. According to his embodiment, the inspecting tool isrealized in the same overlay metrology, as described above.

With reference to the previously-described FIG. 9 which illustrates Hemain optical elements in the measuring unit 22, only a few opticalelements need to be added in addition to those used in the overlaymetrology system as described above to enable the apparatus also to beused for inspecting of defects. Thus, the same apparatus can serve bothas an overlay metrology tool or as a defect inspecting tool. Obviously,the defect inspecting functions can be realized in a separate apparatusthan the overlay metrology function.

The additional elements added to the measuring unit 22 of FIG. 9, forthe defect inspecting function, include a light source 300, shutters 302and 304 to block either the additional light source 300 or light source32, and a ring light 306 which surrounds the objective 66. Ring light306 has to produce a uniform light cone around the objective 66 with anopening angle of ca. 5-10°.

FIG. 16 illustrates how light from an external light source 300 isconveyed to the ring light 306. This is achieved by means of a bundle308 of optic fibers passing through the sealed enclosure 21, whereineach single fiber leads its light to a certain location onto theringlight. In the illustrated embodiment, ring light 306 is a fiberoptic ring light. As an alternative, LEDs with narrow bandwidths couldbe used when placed along the ring light periphery. The ring light isaimed at producing a uniform light-cone with an opening angle largerthan ca. 2° (α in FIG. 16) in order to cause diffracted non-specularlight from the wafer W to fill the objective 66.

Alternatively, light 310 coming through objective 66 from light source32 illuminates the wafer W and its specular component fills objective66.

Thus, in this preferred embodiment, illumination and viewing methods forcoarse inspecting 62 are alternatively bright (BF) and dark (DF) fieldsilluminations, using shutter means 302 and 304 to block either lightsource, or by turning on/off the electrical supply to the light source.The fine inspecting channel 64 is realized by BF illumination only.

It will be appreciated that illuminating and viewing, in general, can berealized by either BF or DF illuminations, all dependent on specificinspecting goals (e.g., defect type). Also during BF and/or DFillumination, for certain applications increased content can be realizedfor example, by additional filter(s) (not shown) after light sources 32and 300, respectively, in order to produce a certain narrow bandwidth.Further, during DF illumination, and for certain applications, a betterdistinction between diffraction and scattering effects can be achieved,e.g., by alternating broad and narrow spectral band illumination.

The defect inspecting tool may be designed to meet the same principlesdescribed above with respect to an overlay metrology system.

FIG. 17 is a schematic flow chart of a method for both coarse and fineinspecting in accordance with a preferred embodiment of the presentinvention.

After a new wafer to be measured arrives at the supporting plate, thewafer is pre-aligned with respect to its principle axis, in order toparallel the wafer's scribe lines and the CCD's lines. An example for amethod for wafer's alignment disclosed in U.S. patent application Ser.No. 09/097,298, now U.S. Pat. No. 6,038,029. After pre-alignment, finalalignment should take place, and a known method for this purpose basedon its pattern features is disclosed in U.S. Pat. No. 5,682,242. Withrespect to fine inspecting, final alignment is aimed at fine correlationof the predetermined site to be inspected with its pattern storedalready in the data base. Such data base is prepared, among otherthings, during recipe preparation.

After final alignment is conducted, image grabbing is performed duringcoarse inspecting in a step and repeat mode. According to this method,the optical head 24 moves to a predetermined area on the wafer, thenstops and stabilizes and an image is grabbed. The procedure is repeatedby moving to the next predetermined site usually used for waferinspection.

According to the present invention, step and repeat procedure allows fora better performance than using a linear scanning method, e.g., rasterscanning. During raster scanning, the wafer is continuously scanned andimages are simultaneously grabbed This method suffers from severaldrawbacks, such as reduced resolution and blurring along the movementaxis, reduced resolution and inaccuracies due to non-stable velocity ofthe scanner, and non-efficient exploitation of the illumination system.During fine inspecting, images are gabbed at predetermined sitesaccording to the recipe. At the next step, each image is processed inorder to search for defects. This is performed either with absolute orcomparative methods as known in the prior art. The processed data isstored in a data base. When the whole wafer complete a coarseinspecting, or all the predetermined sites are finely-inspected, postprocessing commences, Alternatively, post-processing may be conductedsimultaneously. During post processing, the data can be evaluated andreported at different levels. This can be (a) defects list includingnumbers and coordinates of defects detected on the wafer, or (b) defectslist including coordinates and defects dimensions, or (c) defects listincluding coordinates and defects identification, or (d) morphologicaldefects analysis, e.g., according to local and/or overall waferdistribution, such as radial distribution which may point on poorspinning during coating. This can be followed by (e) photographingcertain defects for an additional processing; (f) attributingautomatically defects to a certain problem source; (g) and reviewingoptions for correcting the defects (all or part). In addition, coarseand fine inspection can be combined. According to the processed resultsof the coarse inspection, fine inspection may be conducted in certainsites on a wafer where it is likely to find (e.g., based on thresholds)certain defects.

The post processing data, which is determined by the processing unit,may be transferred to a general control unit 200 of the photoclustertool. General control unit 200 may use this data for a feedback, orclosed loop control, based on the level the data is processed (e.g.,defect identification, or cause analysis). The feedback may be sent tothe coating or other station which may affect the phototrack 5. Thefeedback may also instruct the inspecting metrology system itself withrespect to its operation (e.g., sampling frequency, sites number to bemeasured on a wafer).

It is evident that these embodiments are superior to a parallel‘stand-alone’ system in general, and to a visual inspecting system.

For certain occasions, where a more detailed inspecting is needed theILM system 14 can be used as an off-line system so as not to disturb theproduction process.

According to another preferred embodiment of the present invention, theabove-described overlay metrology system can also be used as OCDmetrology system. The OCD metrology system as illustrated in FIG. 9would contain the same channels 62 and 64 as the overlay metrologysystem, as well as the other optic elements. OCD determination would beexecuted in a similar way as overlay error determination as shown inFIG. 14 and discussed above. For this purpose, the optical head 24 (FIG.9) would be moved to a predetermined site by the alignment channel 62,and then the measurement channel 64 would be operated in order to imagethe features to be measured.

FIG. 18 illustrates the FOV 301 of the optical head 24 duringmeasurement. In this example, the FOV contains two typical features tobe measured; line width 312 and space 314. It is noted that according tothis method for OCD determination, the FOV 301 should include a set ofidentical features to be measured. If this set is not part of theoriginal features on the wafer, a test site which includes set(s) ofsuch features should first be prepared.

Since the features to be measured are located in the same layer,focusing is conducted similarly to overlay measurements as describedabove, however, only with respect to one layer, except when features tobe measured are in different layers. The feature's shape isreconstructed, in the same manner as for overlay, from its image usingthe LSF (x or y, z) of the optical system. The width of a space isdetermined by its adjacent lines edges which are reconstructed.

If this method, the width of the identical features in the set aredetermined, and by applying statistical calculations (e.g., mean value)the width of a representative feature is calculated. The accuracy ofthis measurement is based on the feature's degree of symmetry, theoptical system, and the number of identical features to be measured. Itis to be noted that usually, only the two latter parameters can beadjusted and prepared in advance, according to specific circumstances,in order achieve the desired accuracy.

The monitoring and control based on OCD is established in the samemanner described above with respect to overlay error determination

FIG. 19 illustrates a modification in die optical system of FIG. 9; andFIG. 20 illustrates a modification in the system of FIG. 10 when usingthe optical system of FIG. 19. In this preferred embodiment, theinternal configuration of FIG. 8 is slightly different. The MU 22includes a sealed enclosure 421 with an optical window 437, where insideare installed the optical head 424 (modified according to FIG. 19 and20), the optical head's positioning means along x,y, axis 38 as well asalong z axis, the calibrating unit 40, and additional electronicfeatures and optic guides (not shown) which enable, respectively,external electric and light supply, as well as communications means (notshown) to the MU 22 with the CPU 28.

In the modified system of FIG. 19, the low-magnification channel 462,and the high-resolution channel 464, as well as the focusing target 479and the LED 491 (corresponding to channels 62, 64, target 79 and LED 91in. FIG. 9) are contained within the movable optical head 424. It shouldbe noted that the low-magnification channel 462 is used either forpositioning the optical head 424 above a pre-selected site on a wafer tobe measured during overlay, fine inspecting and OCD applications, or forcoarse inspecting. The high-magnification channel 464 is used formeasuring during overlay, OCD and fine inspecting applications.

Light from the external light source 432 is conveyed by optical fiber438 and is split into two branches 438 a, 438 b, conveying the lightinto the sealed enclosures 421. Each of branches 438 a and 438 b isselectively controlled by shutters 470 and 471. Inside the sealedenclosure 421 the light from the two branches is conveyed by mirrors tothe diffusers 450, 451 of the low-magnification channel 462 and thehigh-magnification channel 464, respectively.

The low-magnification channel 462 includes a field lens 466, a beamsplitter 468, an imaging lens 472, a folding mirror 474, a beam splitter490, and the CCD 492. The alignment channel 462 has a relatively lowmagnification power, (e.g., ×0.1-1.0.); and the imagining lens 472 has asmall numerical aperture and images a wide field of view (e.g. 20-40mm).

The high-magnification channel 464 comprises an objective 476, a beamsplitter 477, a tube lens 481, a beam splitter 490, and the same CCD492. This channel has a relatively high magnification power (e.g.×20-100); and the objective 476 has a large numerical aperture sincehigh resolution is needed.

In this preferred embodiment, DF illumination is realized by a ringlight430 and light sources, e.g., LFDs, placed along the circumference of thering light 430 and electric wires to operate the ring light 430. Thering light 430 is aimed at producing uniform light-cone with an openingangle larger than ca. 2° in order to cause diffracted non-specular lightfrom the wafer w to fill the imaging lens 472. Light source 430 can beswitched by turning on/of the electricity supply.

If higher accuracy during measurement is needed, a system similar tothat illustrated in FIG. 10 may be used for accurately determines theactual angle between the optical axis 464 and the wafer surface W, FIG.20 illustrates such a system which, in this case, is installed inside ahousing 425 which surrounds the objective 476, and includes an LED 493,two identical mirrors 494 a, 494 b, two identical lenses 496 a, 496 b,and an electronic position sensor 498, corresponding to elements 76, 93,94 a, 94 b, 96 a, 96 b, 98, respectively, illustrated in FIG. 10. Thesystem in FIG. 20 can measure the angle β between the normal ray and theray 497 from which the angle between the optical axis 499 and thewafer's plane can be determined.

The system of FIGS. 19 and 20 are otherwise constructed and operated insubstantially the same manner as described above with respect to FIGS. 9and 10, and utilize the focusing target 479 and calibration unit 40 formeasuring channel 464.

Selection of the positioning mode of operation utilizinglow-magnification channel 462, or the measuring mode of operationutilizing high-magnification channel 464, is realized by operating themechanical shutters 470 and 471. The focused condition for the measuringchannel 464 is determined according to known procedures, such as thosedescribed in the above-cited U.S. Pat. No. 5,604,344.

The modified optical system illustrated in FIG. 19 may also be used fordetermining overlay error in accordance with the flow chartschematically illustrated in FIG. 14. After a new wafer to be measuredarrives at the supporting plate 20, calibration of the measuring systemtakes place by identifying a predetermined site on the wafer W, andlocating the optical head 424 above it. The identification of apredetermined site may be based on the wafer pattern features, asdisclosed in the above-cited U.S. Pat. No. 5,682,242.

The modified optical system illustrated in FIG. 19 may also be used forOCD measurements and inspecting in accordance with the flow chartschematically illustrated in FIG. 17.

Turning now to FIGS. 21A and 21B, there are illustrated measuring units,generally designated 22A and 22B, respectively, according to two moreexamples of the invention. It should be noted that, although in thesespecific examples the measuring units are constructed generally similarto the unit of FIG. 19, namely utilize dark-field inspection mode, theprovision of the dark-field mode is optional. To facilitateillustration, the same reference numbers are used for identifying thosecomponents, which are common in the examples of FIGS. 19, 21A and 21B.

The examples of FIGS. 21A and 21B differ from the previously describedexamples in the provision of a spectrophotometric measurement channelaccommodated so as to operate in a bright-field illumination mode.According to the example of FIG. 21A, the spectrophotometric channel500A is a separate channel, namely utilizes a separate lightdirecting/collecting optics and, optionally, a separate light source591. In the example of FIG. 21B, the spectrophotometric channel 500Butilizes optical elements of the low-magnification channel 462 and thesame light source 491.

Thus, in the example of FIG. 21A, the channel 500A is formed by thepropagation of incident light from a broadband light source 591 (or 432,as the case may be) towards the wafer W through an objective 576 and abeam splitter 577, and the propagation of reflected light towards aspectrophotometer 592A through the objective 576, the beam splitter 577and an tube lens 572.

In the example of FIG. 21B, the spectrophotometric channel 500B isformed by the incident light propagation from the broadband light source432 to the wafer W through the beam splitter 468 and the filed lens 466,and the propagation of returned light from the wafer W towards aspectrophotometer 592B through the elements 466 and 468, and furtherthrough the imaging lens 472, a beam splitter 574 (replacing the foldingmirror 474 of FIG. 19) and an additional lens 576.

When the example illustrated on FIG. 21B is applied, it is clear to onesskilled in the art that so-called “boresight” alignment between thechannel 500B and the other channels should be done prior to operation,e.g. using internal target 42 as shown on FIG. 8 or any other well-knowntechnique.

Data (spectra) generated by the spectrophotometer (502A or 592B) isanalyzed for measurement and inspection purposes. For example, certainparameters of the wafer can be measured, such as the layers' thicknesses(typically, PR-layer thickness), layers' optical parameters like indexof refraction and extinction coefficient (preferably, anti-reflectivecoating (ARC) layer optical parameters utilizing an DUV spectral range),etc. ARC layer, usually is applied underneath the PR layer and sometimesit covers it, Analysis of the spectrophotometic measured data mayutilize any known suitable technique, for example the technique used inNovaScan 210 commercially available from the assignee of the presentapplication. When measuring in patterned structures (e.g., patternedsites of semiconductor wafers), various measurement techniques may beused, for example such as disclosed in U.S. Pat. No. 6,100,985 assignedto the assignee of the present application, or diffraction-effects basedtechnique for critical dimensions (CD) measurements disclosed in aco-pending US patent application assigned to the assignee of the presentapplication, which are therefore incorporated herein by reference.

It should be noted that the spectrophotometric channel may also be usedfor the purposes of the so-called “macro inspection” aimed at estimatingthe quality of the PR coating. A suitable technique is disclosed in aco-pending Israeli patent application No. 138193, corresponding to U.S.patent publication No. 2002/0033450 A1.

The spectrophotometric channel can be used for measuring overlayregistration errors. A suitable technique is disclosed in the co-pendingIsraeli patent application No. 138552, corresponding to WO 02/25723assigned to the assignee of the present application, which is thereforeincorporated herein by references. Generally, this is adiffraction-effects based technique enabling high-accuracy detection ofmisalignment between the layers. The technique utilizes detection andanalysis of spectral changes caused by the lateral shift betweendiffraction-grating based targets provided on the layers.

The spectrophotometric channel can be used together with the inspectionchannel (preferably high-magnification channel) for defects'verification and classification purposes. This is associated with thefact that different defective structures (e.g., including differentmaterials) are characterized by different spectral responses. By using aspecific library (database) of spectral responses of differentmaterials, the character of contamination may for example be determined.

While the invention has been described with respect to several preferredembodiments, it will be appreciated that these are set forth merely forpurposes of example, and that many other variations, modifications andapplications of the invention may be made.

What is claimed is:
 1. An apparatus for processing substrates accordingto a predetermined photolithography process, comprising: a loadingstation in which the substrates are loaded from at least one firstcassette; a coating station in which the substrates are coated with aphotoresist material; an exposing station in which the photoresistcoating is exposed to light through a mask having a predeterminedpattern to produce a latent image of the mask on the photoresistcoating; a developing station in which the latent image is developed; anunloading station in which the substrates are unloaded; and a monitoringstation for monitoring the substrates with respect to predeterminedparameters of said photolithography process before being unloaded in atleast one second cassette at said unloading station, said monitoringstation comprising: a supporting assembly for receiving substrates to beinspected; a sealed enclosure having a transparent window aligned withand facing said supporting plate, said supporting plate being externallyof said sealed enclosure and spaced from said window thereof; and anoptical monitoring system within said sealed enclosure for inspectingsubstrates on said supporting plate via said transparent window, whereinsaid optical monitoring system is associated with at least one lightsource and comprises a spectrophotometric measurement channel, and isconfigured for detecting overlay registration errors in the alignment ofthe developed image produced on the substrate in the respectivephotolithography process with respect to a developed image produced onthe substrate in a preceding photolithography process performed on thesubstrate.
 2. The apparatus according to claim 1, and further comprisinga transfer device for transferring said substrates from one station toanother, and for transferring said substrate to said monitoring station.3. The apparatus according to claim 1, wherein said monitoring stationis accommodated between said developing station and said unloadingstation for monitoring the substrates after the substrates have passedthrough the developing station and before reaching the unloadingstation.
 4. The apparatus according to claim 1, wherein said opticalmonitoring system is configured for detecting defects in the substrate,including defects produced by the photolithography process performed onthe substrate.
 5. The apparatus according to claim 1, wherein saidoptical monitoring system is configured for detecting criticaldimensional errors in the developed image of the photoresist coatingproduced during the respective photolithography process.
 6. Theapparatus according to claim 1, wherein said spectrophotometric channelis operable for measuring parameters of the substrates.
 7. The apparatusaccording to claim 6, wherein said parameters includes at least one ofthe following: thickness of at least one layer of the substrate andoptical parameters of at least one layer of the substrate.
 8. Theapparatus according to claim 1, wherein said spectrophotometric channelis operable for inspecting the substrates.
 9. The apparatus according toclaim 8, wherein the inspection includes estimation of the quality ofthe photoresist pattern.
 10. The apparatus according to claim 1, whereinsaid at least one light source is externally of said sealed enclosureand produces at least one light beam which is applied to said opticalsystem within said sealed enclosure.
 11. The apparatus according toclaim 1, wherein: said optical monitoring system within said sealedenclosure includes an optical imaging device; and said monitoringstation further includes an optical image processing unit externally ofsaid sealed enclosure and connected to said optical imaging device byelectrical conductors passing into said sealed enclosure.
 12. Theapparatus according to claim 1, wherein said monitoring station furtherincludes a central control and processing unit externally of said sealedenclosure and connected to said optical monitoring system forcontrolling said system via electrical conductors passing into saidsealed enclosure.
 13. The apparatus according to claim 1, wherein saidoptical monitoring system within said sealed enclosure includes: alow-magnification channel for aligning said optical monitoring systemwith respect to a substrate on said supporting plate; and ahigh-magnification channel for measuring said predetermined parametersof the photolithography process after the substrate has passed throughsaid developing station and before reaching the unloading station. 14.The apparatus according to claim 13, wherein said optical monitoringsystem within said sealed enclosure includes an optical head movablewithin said sealed enclosure and containing an objective lens for saidlow-magnification channel, and an objective lens for saidhigh-magnification channel.
 15. The apparatus according to claim 14,wherein, said lenses are movable together upon moving said optical head.16. The apparatus according to claim 14, wherein the objective lens ofsaid low-magnification channel has a relatively small numericalaperture, and the objective lens of said high-magnification channel hasa relatively large numerical aperture.
 17. The apparatus according toclaim 13, wherein said high-magnification channel includes a measuringdevice within the optical head for accurately measuring the angle ofincidence of the light source with respect to the surface of thesubstrate in the monitoring station.
 18. The apparatus according toclaim 13, wherein said spectrophotometric channel includes the objectivelens of the low-magnification channel.
 19. The apparatus according toclaim 14, wherein said optical head includes an objective lens of saidspectrophotometric channel.
 20. The apparatus according to claim 13,wherein said spectrophotometric channel includes an objective lens, abeam splitter separating incident light and light coming from thesubstrate, an imaging lens and a spectrophotometer.
 21. The apparatusaccording to claim 13, wherein said spectrophotometric channel includesan objective lens, a beam splitter and an imaging lens, all beingoptical elements of the low-magnification channel.
 22. The apparatusaccording to claim 13, and also comprising at least one additional lightsource, light produced by the at least two light sources propagatingthrough the low- and high-magnification channels, respectively.
 23. Theapparatus according to claim 20, and also comprising two additionallight source, light produced by the three light sources propagatingthrough the low-magnification, high-magnification and spectrophotometricchannels.
 24. A monitoring apparatus for optically monitoring articles,comprising: a supporting plate for supporting the article to bemonitored; a sealed enclosure having a transparent window aligned withand facing said supporting plate, said supporting plate being externallyof said sealed enclosure and spaced from said window thereof; an opticalmonitoring system within said sealed enclosure for inspecting thearticle on said supporting plate via said transparent window, theoptical monitoring system comprising a spectrophotometric channel; and alight source for illuminating the article via said optical monitoringsystem, said light source being externally of said sealed enclosure andproducing a light beam which is applied to said optical monitoringsystem within said sealed enclosure.
 25. The apparatus according toclaim 24, wherein: said optical monitoring system within said sealedenclosure includes an optical imaging device; and said monitoringapparatus further includes an image processing unit externally of saidsealed enclosure and connected to said optical imaging device byelectrical conductors passing into said sealed enclosure.
 26. Theapparatus according to claim 24, wherein said apparatus further includesa central control and processing unit externally of said sealedenclosure and connected to said optical monitoring system within saidsealed enclosure via electrical conductors passing into said sealedenclosure.
 27. The apparatus according to claim 24, wherein said articleis a substrate which has been processed according to a predeterminedphotolithography process.
 28. The apparatus according to claim 24,wherein said optical monitoring system within said sealed enclosureincludes a movable optical head.
 29. The apparatus according to claim28, wherein said movable optical head comprises a low-magnificationchannel and a high-magnification channel.
 30. The apparatus according toclaim 27, wherein said spectrophotometric channel is operable formeasuring parameters of the articles including at least one of thefollowing: thickness of at least one layer of the substrate and opticalparameters of at least one layer of the substrate.
 31. The apparatusaccording to claim 24, wherein said spectrophotometric channel isoperable for inspecting the articles.
 32. The apparatus according toclaim 27, wherein said spectrophotometric channel is operable forinspecting the articles, the inspection including estimation of thequality of a photoresist pattern.
 33. The apparatus according to claim24, wherein said spectrophotometric channel includes an objective lens,a beam splitter separating incident light and light coming from thearticle, an imaging lens and a spectrophotometer.
 34. The apparatusaccording to claim 29, wherein said spectrophotometric channel includesan objective lens, a beam splitter and an imaging lens, all beingoptical elements of the low-magnification channel.
 35. The apparatusaccording to claim 32, wherein said optical monitoring system isconfigured for detecting critical dimensional errors in the developedimage of the photoresist coating produced during the respectivephotolithography process.
 36. An apparatus for processing substratesaccording to a predetermined photolithography process, comprising: aloading station in which the substrates are loaded from at least onefirst cassette; a coating station in which the substrates are coatedwith a photoresist material; an exposing station in which thephotoresist coating is exposed to light through a mask having apredetermined pattern to produce a latent image of the mask on thephotoresist coating; a developing station in which the latent image isdeveloped; an unloading station in which the substrates are unloaded;and a monitoring station for monitoring the substrates with respect topredetermined parameters of said photolithography process before beingunloaded in at least one second cassette at said unloading station, saidmonitoring station comprising: a supporting assembly for receivingsubstrates to be inspected; a sealed enclosure having a transparentwindow aligned with and facing said supporting plate, said supportingplate being externally of said sealed enclosure and spaced from saidwindow thereof; and an optical monitoring system within said sealedenclosure for inspecting substrates on said supporting plate via saidtransparent window, wherein said optical monitoring system is associatedwith at least one light source and comprises a spectrophotometricmeasurement channel, said at least one light source being externally ofsaid sealed enclosure and producing at least one light beam which isapplied to said optical system within said sealed enclosure.
 37. Anapparatus for processing substrates according to a predeterminedphotolithography process, comprising: a loading station in which thesubstrates are loaded from at least one first cassette; a coatingstation in which the substrates are coated with a photoresist material;an exposing station in which the photoresist coating is exposed to lightthrough a mask having a predetermined pattern to produce a latent imageof the mask on the photoresist coating; a developing station in whichthe latent image is developed; an unloading station in which thesubstrates are unloaded; and a monitoring station for monitoring thesubstrates with respect to predetermined parameters of saidphotolithography process before being unloaded in at least one secondcassette at said unloading station, said monitoring station comprising:a supporting assembly for receiving substrates to be inspected; a sealedenclosure having a transparent window aligned with and facing saidsupporting plate, said supporting plate being externally of said sealedenclosure and spaced from said window thereof; an optical monitoringsystem within said sealed enclosure for inspecting substrates on saidsupporting plate via said transparent window, wherein said opticalmonitoring system is associated with at least one light source andcomprises a spectrophotometric measurement channel, a low-magnificationchannel for aligning said optical monitoring system with respect to asubstrate on said supporting plate, and a high-magnification channel formeasuring said predetermined parameters of the photolithography processafter the substrate has passed through said developing station andbefore reaching the unloading station.